Siloxane and siloxane derivatives as encapsulants for organic light emitting devices

ABSTRACT

An organic light emitting device ( 10 ) is provided which is encapsulated by a Siloxane film ( 17 ). This Siloxane film ( 17 ) is applied to the light emitting portion of the diode ( 10 ) providing for protection against contamination, degradation, oxidation and the like. The Siloxane film ( 17 ) carries an optical element, such as a lens ( 18 ) for example. This optical element ( 18 ) is arranged such that the light generated inside the diode ( 10 ) is output through it.

This is divisional application of prior application, Ser. No. 09/154,620filed on Sep. 16, 1998, now U.S. Pat. No. 6,337,381, which is adivisional of U.S. Ser. No. 08/7474,791 filed on Nov. 14,1996, now U.S.Pat. No. 5,855,994, which is a 371 PCT/IB96/00665 published on Jul. 10,1996.

TECHNICAL FIELD

The present invention pertains to organic electroluminescent devices,such as discrete light emitting devices, arrays, displays, and inparticular to the encapsulation of these devices. It furthermore relatesto a method for encapsulating the same.

BACKGROUND OF THE INVENTION

Organic electroluminescence (EL) has been studied extensively because ofits possible applications in discrete light emitting devices, arrays anddisplays. Organic materials investigated so far can potentially replaceconventional inorganic materials in many applications and enable whollynew applications. The ease of fabrication and extremely high degrees offreedom in organic EL device synthesis promises even more efficient anddurable materials in the near future which can capitalize on furtherimprovements in device architecture.

Organic EL light emitting devices (OLEDs) function much like inorganicLEDs. Depending on the actual design, light is either extracted througha transparent electrode deposited on a transparent glass substrate, orthrough a transparent top electrode. The first OLEDs were very simple inthat they comprised only a two to three layers. Recent development ledto organic light emitting devices having many different layers (known asmultilayer devices) each of which being optimized for a specific task.

With such multilayer device architectures now employed, a performancelimitation of OLEDs is the reliability. It has been demonstrated thatsome of the organic materials are very sensitive to contamination,oxidation and humidity. Furthermore, most of the metals used as contactelectrodes for OLEDs are susceptible to corrosion in air or other oxygencontaining environments. A Ca cathode, for example, survives intact onlya short time in air, leading to rapid device degradation. It is alsolikely that such highly reactive metals undergo a chemical reaction withthe nearby organic materials which also could have negative effects ondevice performance. A low work function cathode metal approach requirescareful handling of the device to avoid contamination of the cathodemetal, and immediate, high quality encapsulation of the device ifoperation in a normal atmosphere is desired. Even well encapsulated lowwork function metal contacts are subject to degradation resulting fromnaturally evolved gases, impurities, solvents from the organic LEDmaterials.

Many approaches have been attempted in order to solve the problem ofelectrode instability and degradation. A common approach is the use of alow work function metal subsequently buried under a thicker metalcoating. In this case, pinholes in the metal still provide amplepathways for oxygen and water to reach the reactive metal below, as isdescribed in Y. Sato et al., “Stability of organic electroluminescentdiodes”, Molecular Crystals and Liquid Crystals, Vol. 253, 1994, pp.143-150, for example.

The overall lifetime of current organic light emitting devices islimited. The lack of inert, stable, and transparent encapsulants forstable OLED operation remains a major obstacle to OLED development.Organic LEDs have great potential to outperform conventional inorganicLEDs in many applications. One important advantage of OLEDs and devicesbased thereon is the price since they can be deposited on large,inexpensive glass substrates, or a wide range of other inexpensivetransparent, semitransparent or even opaque crystalline ornon-crystalline substrates at low temperature, rather than on expensivecrystalline substrates of limited area at comparatively higher growthtemperatures (as is the case for inorganic LEDs). The substrates mayeven be flexible enabling pliant OLEDs and new types of displays. Todate, the performance of OLEDs and devices based thereon is inferior toinorganic ones for several reasons:

1. High operating current: Organic devices require more current totransport the required charge to the active region (emission layer)which in turn lowers the power efficiency of such devices.

2. Reliability: Organic LEDs degrade in air and during operation.Several problems are known to contribute.

A) Efficient low field electron injection requires low work functioncathode metals like Mg, Ca, Li etc. which are all highly reactive inoxygen and water. Ambient gases and impurities coming out of the organicmaterials degrade the contacts.

B) Conventional AgMg and ITO contacts still have a significant barrierto carrier injection in preferred ETL and HTL materials, respectively.Therefore, a high electric field is needed to produce significantinjection current.

3. Poor chemical stability: Organic materials commonly used in OLEDs arevulnerable to degradation caused by the ambient atmosphere, diffusion ofcontact electrode material, interdiffusion of organics, and reactions oforganics with electrode materials.

As can be seen from the above description there is a need for simple andefficient encapsulation of organic light emitting devices. It is afurther problem of light emitting devices in general, that a light pathfor emission of the light generated is to be provided.

It is an object of the present invention to provide a simple and cheapencapsulation of organic light emitting devices.

It is a further object of the present invention to provide new andimproved organic EL devices, arrays and displays based thereon withimproved stability and reliability.

It is a further object to provide a method for making the present newand improved organic EL devices, arrays and displays.

SUMMARY OF THE INVENTION

The invention as claimed is intended to improve the reliability of knownorganic light emitting devices. The above objects have been accomplishedby providing a transparent Siloxane or Siloxane derivative encapsulationfor an organic light emitting device. The encapsulant comprises anoptical element being arranged such that it lies within the light pathof the light emitted by said organic light emitting device. Examples ofoptical elements that may be formed in, or embedded by the encapsulantare: lenses, filters, color converters, gratings, prisms and the like.

The present invention builds on the finding that Siloxanes and Siloxanederivatives are well suited for use in direct contact with the organicmaterials used for making organic light emitting devices. This is incontrast to currently accepted OLED technology, where no material isallowed to come into direct contact with the organic device. CurrentOLEDs are protected by ‘mechanical’ sealing, e.g. using an appropriatehousing and sealing means.

In contrast to conventional approaches, the encapsulant is also allowedto cover the light emitting portion(s), or part thereof. It turned outthat Siloxanes and Siloxane derivatives do not seem to have adetrimental impact on the behavior and lifetime of the light emittingportion of organic devices.

The Siloxanes and Siloxane derivatives form a transparent andnon-reactive seal which makes conformal contact with the organicdevices. It provides for an excellent barrier to external contamination,such as water, solvent, dust and the like. The proposed encapsulant alsoprotects against corrosion of the highly reactive metal electrodes (e.g.calcium, magnesium, lithium) used in OLED devices. It is non-conductive,which is of particular importance in case that metal electrodes are alsoembedded in the encapsulant.

Furthermore, Siloxane and Siloxane derivatives are extremely robust andstable. They are unlikely to react with the organic devices even inhigh-driving, high-heating conditions. Even close to the light emittingportion(s) of OLEDs, where usually the power density has its maximum, noreaction with the present encapsulant takes place.

It is another important feature of Siloxane and Siloxane derivativesthat it forms a conformal contact with the underlying organic materialsuch that no air, solvent, or water is trapped. Due to this, thelifetime of the organic device is extended.

Further advantages of the Siloxane and Siloxane derivative encapsulationwill be addressed in connection with the embodiments of the presentinvention.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to thefollowing schematic drawings (it is to be noted that the drawings arenot drawn to scale):

FIG. 1 shows a schematic cross-section of a discrete organic lightemitting device being protected by a Siloxane encapsulant comprising anoptical element, according to the present invention.

FIG. 2 shows a schematic cross-section of a a Siloxane encapsulant,according to the present invention, comprising a pocket-like portioncarrying a lens.

FIG. 3 shows a cross-section of a display or array, according to thepresent invention, comprising two Siloxane layers serving asencapsulants.

FIG. 4 shows a top-view of a display or array, according to the presentinvention, comprising a Siloxane film having a matrix of lenses.

FIG. 5 shows a top-view of a conventional display or array.

FIG. 6 shows a top-view of the display or array of FIG. 5, comprising aSiloxane film having a pathways filled with color conversion dyes,according to the present invention.

FIGS. 7A-C illustrates the fabrication of a Siloxane film, according tothe present invention.

GENERAL DESCRIPTION

Silicone molding compounds have been known for more than twenty yearsand their uses include, among others, the encapsulation of electricaland electronic devices. In particular Siloxane, a silicone resin, iswidely used for the molding of electronic devices, such as integratedcircuits, and the coating of portions of such devices. Typical examplesof Siloxanes are composed of copolymers or blends of copolymers of anycombination of monophenylsiloxane units, diphenylsiloxane units,phenylmethylsiloxane units, dimethylsiloxane units, monomethylsiloxaneunits, vinylsiloxane units, phenylvinylsiloxane units,methylvinylsiloxane units, ethylsiloxane units, phenylethylsiloxaneunits, ethylmethylsiloxane units, ethylvinylsiloxane units, ordiethylsiloxane units.

Depending on the resin composition, the properties of Siloxane can bealtered. Some aspects to be taken into account are: stability againstcrack formation, moisture resistance, coefficient of thermal expansion,elastic modulus and crosslinking methods. Siloxanes that cross link bythe hydrosiliation reaction provide a particularly useful subset ofSiloxane. Siloxanes allowing crosslinking on their exposure to light,are also preferred as when the Siloxane prepolymer contains vinyl oracetylenic groups and a light activated radical initiator, for example.

Examples of Siloxanes and Siloxane derivatives suited as encapsulantsfor organic light emitting devices are those given in U.S. Pat. Nos.4,125,510, 4,405,208, 4,701,482, 4,847,120, 5,063,102 and 5,213,864,5,260,398, 5,300,591, and 5,420,213, for example. It is important tochose a Siloxane which is transparent in the wavelength range of thelight emitted by the OLED to be encapsulated. In the following, the wordSiloxane is used as a synonym for all different kinds of transparentSiloxanes. Other materials can be cured with the Siloxane to furtherenhance a material property. Thus mixtures of two polymers can provideenhancement of the device performance as when one component of theencapsulant contains an oxygen scavenger like an organoplatinum complexor Titanium, or a free radical scavenger like tert butanol or somesimilar molecule. Alternatively, the Siloxane also provides a usefulpassivation layer for transfer of a second polymer layer, especiallywhere the latter requires an aggressive solvent that would otherwiseattack the device, but is effectively blocked by the Siloxane. Thissecond layer can further improve performance by preventing passive oractive diffusion of gases through to the OLED.

In order to avoid contamination of the organic stack of the OLED to beencapsulated, or to prevent metal electrodes from corrosion, it turnedout to be important to have an encapsulant which makes conformal contactwith the devices. Furthermore, it is important that the OLED can beencapsulated without having to heat the OLED or without having to treatit with aggressive chemicals.

Siloxane and Siloxane derivatives can be molded into shapes that can beput on the OLED easily. Due to the elastic properties of Siloxane, iteasily conforms to the OLED surface. It is possible, to roll apre-fabricated Siloxane film onto the OLED. It is an interestingproperty of Siloxane, that several films of Siloxane can be stacked oneach other. Siloxanes are particularly well suited to molding on themicron and submicron scales forming stable patterns (structures) withhigh aspect ratio and facile release properties.

Instead of putting pre-fabricated Siloxane onto the OLED, one maylikewise cover the OLED with a viscous Siloxane composition that can becured using ultraviolet radiation, as for example described in U.S. Pat.5,063,102. If a curable Siloxane composition is used, excessive heatingof the OLED is avoided. This case is particularly desirable formicromolding or embossing where the optical element, e.g. a lightbending element, and the encapsulant are formed on the device in asingle manufacturing step.

A first embodiment of the present invention is illustrated in FIG. 1. Adiscrete organic light emitting device 10 is shown. It comprises anelectrode 12 (cathode) situated on a substrate 11. On top of theelectrode 12 a stack of three organic layers 13-15 is situated. Theorganic layer 13 serves as electron transport layer (ETL) and theorganic layer 15 serves as hole transport layer (HTL). The organic layer14 which is embedded between the two transport layers 13 and 15 servesas electroluminescent layer (EL). In the following, the stack of organiclayers will be referred to as organic region, for sake of simplicity. Inthe present embodiment, the organic region carries the reference number19. On top of the HTL 15, a top electrode (anode) 16 is formed. Theuppermost surface of the device 10 is sealed by a Siloxane film 17. Thisfilm 17 conforms to the device 10. In the present example, the opticalelement embedded in the encapsulant 17 is a lens 18. Siloxane may alsobe used to cover and protect cathode-up structures.

Such a lens 18 can be a discrete optical element, which is embedded inthe encapsulant 17 (as shown in FIG. 1). Likewise, a lens 20 may beplaced in a pocket-like section 21 of an encapsulant 22, asschematically illustrated in FIG. 2. In order to further simplify thepackaging and in order to reduce the costs, a lens might be directlyformed in the Siloxane by means of embossing, for example (see FIG. 3).Here a second layer of Siloxane with a higher refraction index can beadded to enhance the lensing.

A second embodiment is illustrated in FIG. 3. In this Figure, across-section of an organic light emitting array 30 is shown. On top ofa common substrate 31, cathodes 32 are patterned such that each of thelight emitting diodes of the array 30 can be individually addressed. Forsake of simplicity, the organic light emitting diodes are depicted as adark grey layer 33. The layer 33 may comprise a stack of organic layers,for example. On top of the organic layer 33, a transparent orsemi-transparent anode 34 is formed. In order to planarize the array 30,a curable Siloxane encapsulant is poured over the top of the array. Byexposure of the Siloxane to ultraviolet light, a thin Siloxane layer35.1 is formed. This layer 35.1 encapsulates the array 30 and providesfor a planarized top surface.

In a next step, a Siloxane film 35.2, comprising embossed lenses 36, isapplied. This Siloxane film 35.2 may for example be rolled onto thearray 30. The Siloxane film 35.2 and the Siloxane layer 35.1 adhere toeach other. The lenses 36 are aligned to the diodes of the array 30 suchthat light emitted by the diodes passes through the anode 34, theSiloxane 35.1 and 35.2 and the lenses 36 before being emitted into thehalfspace above the array 30. In the present arrangement, the size ofeach of the diodes of the array 30 is mainly defined by the shape of thecathodes 32. It is to be noted, that the present invention is alsosuited for cathode-up structures.

In FIG. 4, another embodiment is illustrated. Shown is the top view ofan organic display 40. Only the uppermost layer 43 of this display 40 isvisible in FIG. 4. The display 40 comprises 9×5 rectangular pixels 41.Part of the display's surface is covered and encapsulated by a Siloxanefilm 44. The Siloxane film 44 carries a matrix of lenses 42. The film 44is laterally aligned such that the lenses 42 are aligned with respect tothe pixels 41 such that light is emitted through the lenses 42. Eithermicro or macro lenses may be employed to improve the directionality ofthe light emitted, or to focus the light. Focussing is for examplerequired in a head mounted display. A Siloxane film with macro lensescould be applied to a conventional organic light emitting display tofocus the light of the display on the viewer's eye(s). A Siloxane filmwith integrated optical elements, according to the present invention,eliminates the need for separate optics and combines the encapsulantwith essential device operational items. It can be aligned with respectto the organic device underneath, using an alignment scheme as describedin the co-pending PCT patent application “Stamp for a LithographicProcess”, application number PCT/IB95/00609, filed on Aug. 4, 1995, forexample.

Another organic display embodiment 50 is illustrated in FIG. 5. As inFIG. 4, only the uppermost layer 55 of the display 50 is shown. Thedisplay comprises 9 columns and 11 rows of rectangular pixels 51. Thedisplay 50 is designed such that each pixel 51 emits white light ifdriven accordingly. In order to realize a multicolor display, usuallycolor filters or color converters are employed. According to the presentinvention, no photolithographic steps with undesired chemicals areneeded. In certain cases, photolithography can not be avoided for thedefinition of optical elements. In such a case, one may deposit or forma Siloxane encapsulation, before the photolithographic steps are carriedout. This Siloxane layer then protects the organic device from theaggressive chemical photolithographic steps that may compromise thedevice. An appropriate color converter placed on top of the display 50is illustrated in FIG. 6. Optical elements 53, 54 serving as colorconverters are integrated into a Siloxane film 52. Pathways 53 and 54are provided in the Siloxane film 52 by micromolding or embossing theSiloxane. The depth of the pathways is designed around the colorconversion material, but is typically in the range of 0.1 to 50 μm, andpreferably between 1 and 15 μm. Instead of pathways, containers can beformed within the Siloxane which may have almost any form and size. Thepathways 53 contain a first color converting dye and the pathways 54contain another color converting dye. In the present example, theSiloxane film 52 is placed on top of the display 50 such that the first,fourth and seventh column of pixels is aligned to the pathways 53. Thesecond, fifth and eights column of pixels is aligned to the pathways 54.By choosing appropriate color converting dyes, a three-color display canbe realized.

Such color converters can be easily made, as briefly described in thefollowing. In FIG. 7A, a the Siloxane film 52 is shown which comprises afirst set of pathways 53 and a second set of pathways 54. In order tofill the first set of pathways 53 with a color conversion dye givinggreen light, for example, one edge of the Siloxane film 52 may be dippedinto a bath 70 comprising a suited dye, as shown in FIG. 7B. The dye isnow automatically loaded into the pathways 53 by capillary action. Ifthe first set of pathways 53 is filled, the Siloxane film 52 is flippedand the opposite edge is dipped into another bath 71 comprising anotherdye. By sealing the capillary opening with Siloxane, for example, thedye(s) can remain in a solution state, further enhancing spectralperformance and efficiency of the color conversion dye. Likewise, onemay allow the liquid to evaporate leaving the solid dye confined. Bydoing so, the second set of pathways 54 is filled with the dye containedin the second bath 71 (see FIG. 7C). The second dye may be a dye givingred. The dyes can remain a liquid within the pathways provided in theSiloxane film 52, or its solvents can drain away leaving the dye behindin a solid state.

By means of the above approach, a blue emitting organic array 50 may bepatterned by red and green color converters thereby giving a full-colorRGB (red, green, blue) display, as shown in FIG. 6.

A Siloxane film or encapsulant can be easily mass-fabricated. Therespective fabrication steps can be carried out independently withouthaving a detrimental effect on the more complicated OLED device.

Over a broad sheet of organic light emitters, patterns in a Siloxanefilm may be filled with color conversion dyes by capillary action so asto form multicolor static images. These images may be modified byreplacement of the patterned Siloxane film with the encapsulated dye, orby a micro-fluidic manipulation of the color conversion dyes in theSiloxane pattern. The pathways in the Siloxane film are filled withimmiscible liquids, each filled with a dye corresponding to the desiredcolor. These pathways can then be filled or emptied by approximateapplication of pressure. or other means that causes the liquid to flowin or out of the pathways.

Depending on the composition and thickness of the Siloxane used, aflexible encapsulant can be obtained. Such a flexible encapsulant can beapplied to organic light emitting devices being formed on a flexiblesubstrate. It is possible, for instance, to realize flexible organicdisplays being protected by a flexible encapsulant.

Examples of optical elements that may be formed in, or embedded by theencapsulant are: lenses, filters, color converters, gratings, diffusers,polarizers, and prisms just to mention some examples. A mixture of colorconverters and attenuators may be brought into contact with, or formedon top of an organic multi-color light emitting array, in order tocompensate for unequal efficiency of the light generation at differentwavelengths. It is also feasible to form a Siloxane film comprisingtrapped bubbles. These bubbles serve as optical elements which interactwith the light emitted by the OLED underneath. A lens can be easilyformed by providing an empty bubble of well defined size and shapewithin the Siloxane. This can for example be achieved by embedding asample of the respective size and shape in Siloxane. An appropriatesample should be chosen such that it can be easily removed later. It isconceivable to remove it using an etchant, or by solving it in asuitable solvent. Likewise, it may be removed mechanically.

To summarize, the above exemplary embodiments are fully compatible withany kind of organic light emitting devices, including polymeric,oligomeric, and small molecule OLED designs, or any hybrid designthereof.

What is claimed is:
 1. A method of making a Siloxane film for protectionof part of the light emitting portion of an emitting device's contactelectrode, said Siloxane film having integrated color conversionelements or color filters, said method comprising the steps of: formingone or more pathways for containing dyes in said Siloxane film, andfilling said pathways with a dye.
 2. The method of claim 1, wherein eachof said pathways has a color converting dye independently selected fromthe group consisting of: a first color converting dye and a second colorconverting dye.
 3. The method of claim 1, wherein said dye is a colorconversion dye.
 4. The method of claim 1, wherein said dye is either inits liquid state or in its solid state.
 5. A method of making a Siloxanefilm for encapsulating an optical element selected from a lens, filter,color converter, grating, prism, diffuser, polarizer and a combinationthereof, said method comprising the steps of: applying onto said opticalelement a curable Siloxane composition comprising a compound having oneor more ethylenically or acetylenically unsaturated groups, ahydrosilylating agent and a radical initiator, and curing said curablecomposition.
 6. The method of claim 5, wherein said Siloxane filmcomprises an array or matrix of optical elements.
 7. The method of claim5, wherein said optical element is selected from the group consistingof: a filter, color converter, and a combination thereof.
 8. The methodof claim 5, wherein said optical element is embedded in said Siloxanefilm.
 9. The method of claim 5, wherein said optical element is formedin said Siloxane film.
 10. The method of claim 5, wherein said opticalelement is placed in a pocket portion of said Siloxane film.
 11. Themethod of claim 5, wherein said curable composition comprises a compoundhaving ethylenically unsaturated groups, a hydrosilylating agent and alight activated radical initiator.
 12. The method of claim 5, whereinsaid curing step is initiated by light.